"The Office of Fire Investigation has determined the cause of the 5-11 Fire on Ashland to be open flame ignition of available combustibles," said Fire Department spokesman Larry Langford of the largest fire to hit the city in seven years.

"In layman's terms, that means something that was burning such as a flame or match got near something that would burn," Langford said.

Langford said the building had no gas or electric service and no one was known to be living there. But the night of the blaze, Fire Commissioner Jose Santiago told reporters that in the past, firefighters had been called to the warehouse for small fires caused when squatters lit fires to stay warm.

The matter has been forwarded to the Chicago Police Bomb and Arson unit to determine if the fire involved foul play, Langford and police said.

Friday morning, a private wrecking company hired by the owners of the building began its demolition, Chicago Fire Department spokeswoman Meg Ahlheim said.

Tuesday night, a fire department battalion chief spotted smoke from the blaze as he drove past around 9 p.m. A third of the department's on-duty personnel were called to fight a fire fed by century-old support timbers. Crews have remained there since, dousing flames from the smouldering debris.